The present invention relates generally to a control system for compressors operating in parallel. Specifically, the present invention relates to a control system that re-establishes the stability of dual centrifugal compressors operating in parallel upon one of the centrifugal compressors entering into an unstable operating condition such as a surge condition.
To obtain increased capacity in a refrigeration system, two compressors can be connected in parallel to a common refrigerant circuit. Frequently, for capacity control, one of the compressors is designated as a xe2x80x9cleadxe2x80x9d compressor and the other compressor is designated as a xe2x80x9clagxe2x80x9d compressor. The capacity of the refrigeration system, and of each compressor, can be controlled by the use of adjustable pre-rotation vanes or inlet guide vanes incorporated in or adjacent to the suction inlet of each compressor. Depending on the particular capacity requirements of the system, the pre-rotation vanes of each compressor can be positioned to control the flow of refrigerant through the compressors and thereby control the capacity of the system. The positions of the pre-rotation vanes can range from a completely open position to a completely closed position. The pre-rotation vanes for a compressor can be positioned in a more open position to increase the flow of refrigerant through the compressor and thereby increase the capacity of the system or the pre-rotation vanes of a compressor can be positioned in a more closed position to decrease the flow of refrigerant through the compressor and thereby decrease the capacity of the system.
One frequently used method to control the capacity of a refrigeration system is to control the position of the pre-rotation vanes of a compressor in response to a deviation from a desired set point of the leaving chilled water temperature in the evaporator. For a system with two parallel compressors, the pre-rotation vanes of the lead compressor are controlled based on the leaving chilled water temperature and the pre-rotation vanes of the lag compressor are controlled to follow the capacity of the lead compressor. In one technique, to follow the capacity of the lead compressor, the pre-rotation vanes of the lag compressor are positioned to obtain the same percentage of full-load motor current in the lag compressor that is present in the lead compressor.
During the operation of centrifugal compressors, a compressor instability or surge can occur in a centrifugal compressor. Surge or surging is an unstable condition that may occur when compressors, such as centrifugal compressors, are operated at light loads and high pressure ratios. Surge is a transient phenomenon having high frequency oscillations in pressures and flow, and, in some cases, the occurrence of a complete flow reversal through the compressor. Surging, if uncontrolled, can cause excessive vibrations in both the rotating and stationary components of the compressor, and may result in permanent compressor damage. During a surge condition there can exist a momentary reduction in flow and pressure developed across the compressor. Furthermore, there can be a reduction in the net torque and mechanical power at the driving shaft of the compressor. In the case where the drive device of the compressor is an electric motor, the oscillations in torque and power caused by a surge condition can result in oscillations in motor current and excessive electrical power consumption.
As discussed above, a surge condition in a centrifugal compressor can result in a reduction in motor current or load on the compressor or a reduction in discharge pressure or temperature from the compressor. Thus, the presence of a surge condition can be detected by measuring the motor current or load on the compressor or the discharge pressure or temperature from the compressor and checking for the appropriate reduction in the measured amount. It is to be understood that other operational parameters, in addition to the ones discussed above, can be used to detect the presence of a surge condition.
When a surge or lack of pumping condition occurs on one compressor in dual compressor applications, the compressor which does not surge has an increase in refrigerant flow. The increase in refrigerant flow to the non-surging compressor makes it more difficult for the surging compressor to overcome the instability. One technique for overcoming a surge condition in a dual compressor configuration is disclosed in U.S. Pat. No. 4,646,530, hereafter referred to as the U.S. Pat. No. ""530 . The U.S. Pat. No. ""530 is directed to the operation of a refrigeration system having a pair of centrifugal compressors connected in parallel. During a surge condition in the lag compressor, the control operation of the compressors is changed from the normal control operation to a surge control operation. In the U.S. Pat. No. ""530 , a surge condition is detected when the motor current of the lag compressor is more than a selected percentage below the lead compressor motor current. If a surge condition is detected to be present for a predetermined period of time, the inlet guide vanes to the lead compressor are closed for another predetermined period of time to increase the flow of refrigerant and current in the lag compressor. If the current in the lag compressor increases above the selected percentage, after the predetermined time period for the closing of the vanes of the lead compressor, normal control operation of the compressors is resumed. One drawback of this technique is that it can only detect and correct a surge condition in the lag compressor and does not address a surge condition in the lead compressor. Another drawback of this technique is that a predetermined time has to elapse before a response to the surge condition is provided.
Another technique for controlling surge in a dual compressor arrangement is disclosed in U.S. Pat. No. 5,845,509 hereafter referred to as the U.S. Pat. No. ""509 . The U.S. Pat. No. ""509 is directed to a refrigeration system using a plurality of centrifugal compressors operated in parallel. To avoid surge in a two compressor system, the lag compressor is initially shut off in a reduced load situation to thereby increase the rotational speed of the other compressor and avoid a surge condition. However, if load conditions continue to decrease and the surge condition has not been avoided, the lag compressor is re-started and the lead compressor is shut down to attempt to avoid the surge condition. One drawback of this technique is that the compressors can be cycled on and off several times in attempting to avoid surge conditions thereby resulting in significant power consumption.
Therefore, what is needed is a control system and method for dual centrifugal compressors operated in parallel that can detect a surge condition in either the xe2x80x9cleadxe2x80x9d compressor or the xe2x80x9clagxe2x80x9d compressor and can correct the surge condition in the compressor without a complex procedure or repeated on-off cycling of compressors.
One embodiment of the present invention is directed to a method for detecting compressor instability in a multiple compressor refrigeration system. The method includes the steps of determining an operating parameter from both a first compressor of a multiple compressor refrigeration system and a second compressor of the multiple compressor refrigeration system. The operating parameter of the first compressor is then compared to the operating parameter of the second compressor. Next, an inlet vane position for both the first compressor and the second compressor is determined. Finally, the inlet vane position of the first compressor is compared to the inlet vane position of the second compressor and a compressor instability is determined in one of the compressors in response to that compressor having both a lower operating parameter and a more open inlet vane position than the other compressor.
Another embodiment of the present invention is directed to a computer program product embodied on a computer readable medium and executable by a microprocessor for detecting a compressor instability in a multiple compressor refrigeration system. The computer program product includes computer instructions for executing the steps of determining an operating parameter from both a first compressor of a multiple compressor refrigeration system and a second compressor of the multiple compressor refrigeration system, calculating a reference value using the operating parameter of the first compressor and the operating parameter of the second compressor, and comparing the calculated reference value to a predetermined value. The computer program product also includes computer instructions for executing the steps of determining an inlet vane position for both the first compressor and the second compressor, comparing the inlet vane position of the first compressor to the inlet vane position of the second compressor in response to the calculated reference value being less than the predetermined value, and determining a compressor instability in one of the first compressor and the second compressor in response to the one of the first compressor and the second compressor having both a lower operating parameter and a more open inlet vane position than the other compressor of the first compressor and the second compressor.
Still another embodiment of the present invention is directed to a stability control system for a refrigeration system comprising a lead compressor, a lag compressor, a condenser, and an evaporator connected in a closed refrigeration circuit. The lead compressor and the lag compressor each have a plurality of inlet guides vanes adjustable by an actuator. The stability control system including a first sensor configured and disposed to detect an operating parameter of the lead compressor and to generate a first signal corresponding to the detected operating parameter of the lead compressor, a second sensor configured and disposed to detect a position of the plurality of inlet guide vanes of the lead compressor and to generate a second signal corresponding to the detected position of the plurality of inlet guide vanes of the lead compressor, a third sensor configured and disposed to detect an operating parameter of the lag compressor and to generate a third signal corresponding to the detected operating parameter of the lag compressor, and a fourth sensor configured and disposed to detect a position of the plurality of inlet guide vanes of the lag compressor and to generate a fourth signal corresponding to the detected position of the plurality of inlet guide vanes of the lag compressor. The stability control system also includes a microprocessor configured to receive the first signal, the second signal, the third signal and the fourth signal during normal operation of the refrigeration system, and to generate control signals for the actuators of the plurality of inlet guide vanes of the lead compressor and the lag compressor by applying the first signal, the second signal, the third signal and the fourth signal to a control algorithm configured to determine a surge condition in one of the lead compressor and the lag compressor.
One advantage of the present invention is that it can detect and control surge in either compressor of a dual compressor system.
Another advantage of the present invention is that corrective control responses can be taken in response to the detection of an unstable operating condition without a significant time delay.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.